A space-based mobile telecommunication system is unique from ground-based or terrestrial cellular systems in at least three ways. First, the movement of subscribers from cell to cell is dictated by predictable motions of the satellites and not the movement of individual subscribers as in ground-based telecommunication systems.
Second, it is unique in the aspect of system outages and subsequent repair. A ground-based telecommunication system is accessible by field maintenance personnel who can fix problems as they arise. In contrast, a space-based mobile telecommunication system is prohibitively expensive and impractical to repair by traditional means. When a problem arises on a satellite, someone may have to decide whether the degree of the impairment is greater than the cost of decommissioning the entire satellite and replacing it with a new one. Even if another satellite is available, it may take a significant amount of time to maneuver the spare satellite into the orbit to replace the nonfunctioning satellite. Therefore, there will be coverage holes in the normally global communication coverage provided by the space-based mobile telecommunication system when a satellite becomes non-operational.
Third, space-based mobile telecommunication systems have frequent frequency replanning. In ground-based systems, frequency replanning is often measured on a scale of months. Space-based systems, however, are forced into frequency replanning after a few seconds because of the constantly changing relationship of the satellites to each other and to earth. One of the disadvantages of frequent frequency replanning is that cells undergoing frequency replanning may find that the new frequency plan does not support as much capacity as the old plan. In some cases, this may lead to some calls being terminated for lack of call capacity.
For these reasons, there is a significant need for methods which prewarn a subscriber that its call will shortly be dropped due either to a non-functioning satellite or a reduction in call capacity resources.